Filter for use with imaging endoscopes

ABSTRACT

A fluorescence endoscopy video system includes a multi-mode light source that produces light for white light and fluorescence imaging modes. A filter is positioned at the distal end of an imaging endoscope so that the endoscope can produce fluorescence and white light images of a tissue sample.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/009,965, filed Dec. 10, 2004, which is a continuation ofU.S. patent application Ser. No. 10/050,601, filed Jan. 15, 2002, thebenefit of the filing dates of which are being claimed under 35 U.S.C. §120.

FIELD OF THE INVENTION

The present invention relates to medical imaging systems in general, andin particular to fluorescence endoscopy video systems.

BACKGROUND OF THE INVENTION

Fluorescence endoscopy utilizes differences in the fluorescence responseof normal tissue and tissue suspicious for early cancer as a tool in thedetection and localization of such cancer. The fluorescing compounds orfluorophores that are excited during fluorescence endoscopy may beexogenously applied photo-active drugs that accumulate preferentially insuspicious tissues, or they may be the endogenous fluorophores that arepresent in all tissue. In the latter case, the fluorescence from thetissue is typically referred to as autofluorescence or nativefluorescence. Tissue autofluorescence is typically due to fluorophoreswith absorption bands in the UV and blue portion of the visible spectrumand emission bands in the green to red portions of the visible spectrum.In tissue suspicious for early cancer, the green portion of theautofluorescence spectrum is significantly suppressed. Fluorescenceendoscopy that is based on tissue autofluorescence utilizes thisspectral difference to distinguish normal from suspicious tissue.

Since the concentration and/or quantum efficiency of the endogenousfluorophores in tissue is relatively low, the fluorescence emitted bythese fluorophores is not typically visible to the naked eye.Fluorescence endoscopy is consequently performed by employing low lightimage sensors to acquire images of the fluorescing tissue through theendoscope. The images acquired by these sensors are most often encodedas video signals and displayed on a color video monitor. Representativefluorescence endoscopy video systems that image tissue autofluorescenceare disclosed in U.S. Pat. No. 5,507,287, issued to Palcic et al.; U.S.Pat. No. 5,590,660, issued to MacAulay et al.; U.S. Pat. No. 5,827,190,issued to Palcic et al., U.S. patent application Ser. No. 09/615,965,and U.S. patent application Ser. No. 09/905,642, all of which are hereinincorporated by reference. Each of these is assigned to XillixTechnologies Corp. of Richmond, British Columbia, Canada, the assigneeof the present application.

While the systems disclosed in the above-referenced patents aresignificant advances in the field of early cancer detection,improvements can be made. In particular, it is desirable to reduce thesize, cost, weight, and complexity of the camera.

SUMMARY OF THE INVENTION

A fluorescence endoscopy video system in accordance with the presentinvention includes an endoscopic light source that is capable ofoperating in multiple modes to produce either white light, reflectancelight, fluorescence excitation light, or fluorescence excitation lightwith reference reflectance light. An endoscope incorporates a lightguide for transmitting light to the tissue under observation andincludes either an imaging guide or a compact camera disposed in theinsertion portion of the endoscope for receiving light from the tissueunder observation. A compact camera includes at least one low lightimaging sensor that receives light from the tissue and is capable ofoperating in multiple imaging modes to acquire color or multi-channelfluorescence and reflectance images. The system further includes animage processor and system controller that digitizes, processes andencodes the image signals produced by the image sensor(s) as a colorvideo signal and a color video monitor that displays the processed videoimages.

In one embodiment of the invention, a filter is placed at the distal endof the endoscope in order to produce both autofluorescence and whitelight images from an image sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1B are block diagrams of a fluorescence endoscopy video systemaccording to one embodiment of the present invention;

FIGS. 2A-2B are block diagrams of a multi-mode light source inaccordance with different embodiments of the present invention;

FIG. 3 shows a filter wheel and optical filters for the multi-mode lightsource;

FIGS. 4A-4C illustrate a number of alternative embodiments of a camerathat can acquire color and/or fluorescence/reflectance images accordingto one embodiment of the present invention with optional placement forcollimation and imaging optics;

FIGS. 5A-5C illustrate a number of camera beamsplitter configurations;

FIGS. 6A-6E are graphs illustrating presently preferred transmissioncharacteristics of filters utilized for color imaging andfluorescence/reflectance imaging with the camera embodiments shown inFIGS. 4A-4C;

FIGS. 7A-7B illustrate additional embodiments of a camera according tothe present invention that can acquire color, fluorescence/reflectance,and/or fluorescence/fluorescence images according to an embodiment ofthe present invention with optional placement for collimation andimaging optics;

FIGS. 8A-8F are graphs illustrating presently preferred transmissioncharacteristics of filters for color imaging, fluorescence/fluorescenceimaging, and fluorescence/reflectance imaging with the camera embodimentshown in FIGS. 7A-7B;

FIG. 9 illustrates a distal end filter in accordance with anotherembodiment of the present invention that allows a conventional endoscopeto perform both fluorescence and white light imaging;

FIGS. 10A and 10B illustrate how the distal end filter allows theendoscope to perform fluorescence and white light imaging in accordancewith one embodiment of the present invention; and

FIG. 11 illustrates one embodiment of a distal end filter that can besecured to a conventional endoscope in accordance with the presentinvention.

DETAILED DESCRIPTION

FIG. 1A is a block diagram of a fluorescence endoscopy video system 50in accordance with one embodiment of the present invention. The systemincludes a multi-mode light source 52 that generates light for obtainingcolor and fluorescence images. The use of the light source for obtainingdifferent kinds of images will be described in further detail below.Light from the light source 52 is supplied to an illumination guide 54of an endoscope 60, which then illuminates a tissue sample 58 that is tobe imaged.

As shown in FIG. 1A, the system also includes a multi-mode camera 100,which is located at the insertion end of the endoscope 60. The lightfrom the tissue is directly captured by the multi-mode camera 100. Withthe multi-mode camera 100 located at the insertion end of the endoscope,the resulting endoscope 60 can be characterized as a fluorescence videoendoscope, similar to video endoscopes currently on the market (such asthe Olympus CF-240L) in utility, but with the ability to be utilized forfluorescence/reflectance and/or fluorescence/fluorescence imaging, inadditional to conventional color imaging. Fluorescence/reflectance andfluorescence/fluorescence imaging will be described in detail below. Bylocating the camera at the insertion end of the endoscope, the inherentadvantages of a video endoscope can be obtained: namely, the lightavailable to form an image and the image resolution are improvedcompared to the case when the image is transmitted outside the bodythrough an endoscope imaging guide or relay lens system.

A processor/controller 64 controls the multi-mode camera 100 and thelight source 52, and produces video signals that are displayed on avideo monitor 66. The processor/controller 64 communicates with themulti-mode camera 100 with wires or other signal carrying devices thatare routed within the endoscope. Alternatively, communication betweenthe processor/controller 64 and the camera 100 can be conducted over awireless link.

FIG. 1B is a block diagram of an alternative fluorescence endoscopyvideo system 50, which differs from that shown in FIG. 1A in thatendoscope 60 also incorporates an image guide 56 and the multi-modecamera 100 is attached to an external portion of the endoscope that isoutside the body. The light that is collected from the tissue byendoscope 60 is transmitted through the image guide 56 and projectedinto the multi-mode camera 100. Other than the addition of the imageguide 56 to endoscope 100 and the location of the multi-mode camera 100at the external end of the endoscope, the system of FIG. 1B is identicalto that shown in FIG. 1A.

FIG. 2A shows the components of the light source 52 in greater detail.The light source 52 includes an arc lamp 70 that is surrounded by areflector 72. In the preferred embodiment of the invention, the arc lamp70 is a high pressure mercury arc lamp (such as the Osram VIP R150/P24). Alternatively, other arc lamps, solid state devices (such aslight emitting diodes or diode lasers), or broadband light sources maybe used, but a high pressure mercury lamp is currently preferred for itscombination of high blue light output, reasonably flat whitelight-spectrum, and small arc size.

The light from the arc lamp 70 is coupled to a light guide 54 of theendoscope 60 through appropriate optics 74, 76, and 78 for lightcollection, spectral filtering and focusing respectively. The light fromthe arc lamp is spectrally filtered by one of a number of opticalfilters 76A, 76B, 76C . . . that operate to pass or reject desiredwavelengths of light in accordance with the operating mode of thesystem. As used herein, “wavelength” is to be interpreted broadly toinclude not only a single wavelength, but a range of wavelengths aswell.

An intensity control 80 that adjusts the amount of light transmittedalong the light path is positioned at an appropriate location betweenthe arc lamp 70 and the endoscope light guide 54. The intensity control80 adjusts the amount of light that is coupled to the light guide 54. Inaddition, a shutter mechanism 82 may be positioned in the same opticalpath in order to block any of the light from the lamp from reaching thelight guide. A controller 86 operates an actuator 77 that moves thefilters 76A, 76B or 76C into and out of the light path. The controller86 also controls the position of the intensity control 80 and theoperation of the shutter mechanism 82.

The transmission characteristics of filters 76A, 76B, 76C, . . . , thecharacteristics of the actuator 77 mechanism, and the time available formotion of the filters 76A, 76B, 76C, . . . , into and out of the lightpath, depend on the mode of operation required for use with the variouscamera embodiments. The requirements fall into two classes. If the lightsource shown in FIG. 2A is of the class wherein only one filter isutilized per imaging mode, the appropriate filter is moved in or out ofthe light path only when the imaging mode is changed. In that case, theactuator 77 only need change the filter in a time of approximately 1.0second. The optical filter characteristics of filters 76A, 76B . . . aretailored for each imaging mode. For example, optical filter 76A, usedfor color imaging, reduces any spectral peaks and modifies the colortemperature of the arc lamp 70 so that the output spectrum simulatessunlight. Optical filter 76B transmits only fluorescence excitationlight for use with the fluorescence/fluorescence imaging mode andoptical filter 76C transmits both fluorescence excitation light andreference reflectance light for use with the fluorescence/reflectanceimaging mode.

A light source 52A of a second class is illustrated in FIG. 2B; only thedifferences from the light source shown in FIG. 2A will be elucidated.The light source 52A uses multiple filters during each imaging mode. Forexample, light source filters, which provide red, green, and blueillumination sequentially for periods corresponding to a video frame orfield, can be used for the acquisition of a color or a multi-spectralimage with a monochrome image sensor, with the different wavelengthcomponents of the image each acquired at slightly different times. Suchrapid filter changing requires a considerably different actuator thannecessitated for the light source 52 of FIG. 2A. As shown in FIG. 2B,the filters are mounted on a filter wheel 79 that is rotated by a motor,which is synchronized to the video field or frame rate. The layout ofthe blue, red and green filters, 79A, 79B, and 79C respectively, infilter wheel 79 are shown in FIG. 3.

The transmission characteristics of light source filters, thecharacteristics of the filter actuator mechanism, and the time availablefor motion of the filters into and out of the light path, for the twodifferent classes of light sources are described in more detail below inthe context of the various camera embodiments.

Because fluorescence endoscopy is generally used in conjunction withwhite light endoscopy, each of the various embodiments of the multi-modecamera 100 described below may be used both for color andfluorescence/reflectance and/or fluorescence/fluorescence imaging. Thesecamera embodiments particularly lend themselves to incorporation withina fluorescence video endoscope due to their compactness and theirability to be implemented with no moving parts.

In a first embodiment, shown in FIG. 4A, a camera 100A receives lightfrom the tissue 58, either directly from the tissue in the case of acamera located at the insertion end of an endoscope, as shown in FIG.1A, or by virtue of an endoscope image guide 56, which transmits thelight from the tissue to the camera, as shown in FIG. 1B. The light isdirected towards a monochrome image sensor 102 and a low light imagesensor 104 by a fixed optical beamsplitter 106 that splits the incominglight into two beams. The light beam is split such that a smallerproportion of the light received from the tissue 58 is directed towardsthe monochrome image sensor 102 and a larger proportion of the incominglight is directed towards the low light image sensor 104. In thisembodiment, the beamsplitter may be a standard commercially availablesingle plate 88, single cube 89, or single pellicle design 90, as shownin FIGS. 5A-5C. It should be noted that, if the optical path between thetissue 58 and the image sensors contains an uneven number of reflections(e.g., such as from a single component beamsplitter), the imageprojected onto the sensor will be left-to-right inverted. Theorientation of such images will need to be corrected by imageprocessing.

In FIG. 4A, light collimating optics 110 are positioned in front of thebeamsplitter 106, and imaging optics 112 and 114 are positionedimmediately preceding the monochrome image sensor 102 and the low lightimage sensor 104, respectively. A spectral filter 118 is located in theoptical path between the beamsplitter 106 and the low light image sensor104. Alternatively, the spectral filter 118 may be incorporated as anelement of the beamsplitter 106.

FIG. 4B illustrates another embodiment of the camera 100. A camera 100Bis the same as the camera 100A described above except that the lightcollimating optics 110 and imaging optics 112 and 114 have beeneliminated and replaced with a single set of imaging optics 113 locatedbetween the tissue and beamsplitter 106. The advantage of thisconfiguration is that all imaging is performed and controlled by thesame imaging optics 113. Such a configuration requires all beam paths tohave the same optical path length, however, and this restriction must beconsidered in the design of the beamsplitter 106 and spectral filter 118that is located in the path to the low light image sensor 104. Inaddition, the fact that these optical elements are located in aconverging beam path must be considered in specifying these elements andin the design of the imaging optics 113.

The low light image sensor 104 preferably comprises a charge coupleddevice with charge carrier multiplication (of the same type as the TexasInstruments TC253 or the Marconi Technologies CCD65), electron beamcharge coupled device (EBCCD), intensified charge coupled device (ICCD),charge injection device (CID), charge modulation device (CMD),complementary metal oxide semiconductor image sensor (CMOS) or chargecoupled device (CCD) type sensor. The monochrome image sensor 102 ispreferably a CCD or a CMOS image sensor.

An alternative configuration of the camera 100B is shown in FIG. 4C. Allaspects of this embodiment of this camera 100C are similar to the camera100B shown in FIG. 4B except for differences which arise from reducingthe width of the camera by mounting both image sensors 102 and 104perpendicular to the camera front surface. In this alternativeconfiguration, the low light image sensor 104 and the monochrome imagesensor 102 are mounted with their image planes perpendicular to theinput image plane of the camera. Light received from the tissue 58 isprojected by imaging optics 113 through beamsplitter 106 onto the imagesensors 102 and 104. The beamsplitter 106 directs a portion of theincoming light in one beam towards one of the sensors 102, 104. Anotherportion of the incoming light in a second light beam passes straightthrough the beamsplitter 106 and is directed by a mirror 108 towards theother of the sensors 102, 104. In addition, a second set of imagingoptics 115 is utilized to account for the longer optical path to thissecond sensor. The images projected onto both sensors will beleft-to-right inverted and should be inverted by image processing.

The processor/controller 64 as shown in FIGS. 1A and 1B receives thetransduced image signals from the camera 100 and digitizes and processesthese signals. The processed signals are then encoded in a video formatand displayed on a color video monitor 66.

Based on operator input, the processor/controller 64 also providescontrol functions for the fluorescence endoscopy video system. Thesecontrol functions include providing control signals that control thecamera gain in all imaging modes, coordinating the imaging modes of thecamera and light source, and providing a light level control signal forthe light source.

The reason that two separate images in different wavelength bands areacquired in fluorescence imaging modes of the fluorescence endoscopyvideo systems described herein, and the nature of thefluorescence/reflectance and fluorescence/fluorescence imaging, will nowbe explained. It is known that the intensity of the autofluorescence atcertain wavelengths changes as tissues become increasingly abnormal(i.e. as they progress from normal to frank cancer). When visualizingimages formed from such a band of wavelengths of autofluorescence,however, it is not easy to distinguish between those changes in thesignal strength that are due to pathology and those that are due toimaging geometry and shadows. A second fluorescence image acquired in aband of wavelengths in which the image signal is not significantlyaffected by tissue pathology, utilized for fluorescence/fluorescenceimaging, or a reflected light image acquired in a band of wavelengths inwhich the image signal is not significantly affected by tissue pathologyconsisting of light that has undergone scattering within the tissue(known as diffuse reflectance), utilized for fluorescence/reflectanceimaging, may be used as a reference signal with which the signalstrength of the first fluorescence image can be “normalized.” Suchnormalization is described in two patents previously incorporated hereinby reference: U.S. Pat. No. 5,507,287, issued to Palcic et al. describesfluorescence/fluorescence imaging and U.S. Pat. No. 5,590,660, issued toMacAulay et al. describes fluorescence/reflectance imaging.

One technique for performing the normalization is to assign each of thetwo image signals a different display color, e.g., by supplying theimage signals to different color inputs of a color video monitor. Whendisplayed on a color video monitor, the two images are effectivelycombined to form a single image, the combined color of which representsthe relative strengths of the signals from the two images. Since lightoriginating from fluorescence within tissue and diffuse reflectancelight which has undergone scattering within the tissue are both emittedfrom the tissue with a similar spatial distribution of intensities, thecolor of a combined image is independent of the absolute strength of theseparate image signals, and will not change as a result of changes inthe distance or angle of the endoscope 60 to the tissue sample 58, orchanges in other imaging geometry factors. If, however, there is achange in the shape of the autofluorescence spectrum of the observedtissue that gives rise to a change in the relative strength of the twoimage signals, such a change will be represented as a change in thecolor of the displayed image. Another technique for performing thenormalization is to calculate the ratio of the pixel intensities at eachlocation in the two images. A new image can then be created wherein eachpixel has an intensity and color related to the ratio computed. The newimage can then be displayed by supplying it to a color video monitor.

The mixture of colors with which normal tissue and tissue suspicious forearly cancer are displayed depends on the gain applied to each of thetwo separate image signals. There is an optimal gain ratio for whichtissue suspicious for early cancer in a fluorescence image will appearas a distinctly different color than normal tissue. This gain ratio issaid to provide the operator with the best combination of sensitivity(ability to detect suspect tissue) and specificity (ability todiscriminate correctly). If the gain applied to the reference imagesignal is too high compared to the gain applied to the fluorescenceimage signal, the number of tissue areas that appear suspicious, butwhose pathology turns out to be normal, increases. Conversely, if therelative gain applied to the reference image signal is too low,sensitivity decreases and suspect tissue will appear like normal tissue.For optimal system performance, therefore, the ratio of the gainsapplied to the image signals must be maintained at all times. Thecontrol of the gain ratio is described in two patent applicationspreviously incorporated herein by reference: U.S. patent applicationSer. No. 09/615,965, and U.S. patent application Ser. No. 09/905,642.

In vivo spectroscopy has been used to determine which differences intissue autofluorescence and reflectance spectra have a pathologicalbasis. The properties of these spectra determine the particularwavelength bands of autofluorescence and reflected light required forthe fluorescence/reflectance imaging mode, or the particular twowavelength bands of autofluorescence required forfluorescence/fluorescence imaging mode. Since the properties of thespectra depend on the tissue type, the wavelengths of the importantautofluorescence band(s) may depend on the type of tissue being imaged.The specifications of the optical filters described below are aconsequence of these spectral characteristics, and are chosen to beoptimal for the tissues to be imaged.

As indicated above, the filters in the light source and camera should beoptimized for the imaging mode of the camera, the type of tissue to beexamined and/or the type of pre-cancerous tissue to be detected.Although all of the filters described below can be made to order usingstandard, commercially available components, the appropriate wavelengthrange of transmission and degree of blocking outside of the desiredtransmission range for the described fluorescence endoscopy images areimportant to the proper operation of the system. The importance of otherissues in the specification of such filters, such as the fluorescenceproperties of the filter materials and the proper use of anti-reflectioncoatings, are taken to be understood.

FIGS. 6A-6E illustrate the preferred filter characteristics for use in afluorescence endoscopy system having a camera of the type shown in FIGS.4A-4C and light source as shown in FIG. 2B, that operates in afluorescence/reflectance imaging mode, or a color imaging mode. Thereare several possible configurations of fluorescence endoscopy videosystems, operating in the fluorescence/reflectance imaging modeincluding green fluorescence with either red or blue reflectance, andred fluorescence with either green or blue reflectance. The particularconfiguration utilized depends on the target clinical organ andapplication. The filter characteristics will now be described for eachof these four configurations.

FIG. 6A illustrates the composition of the light transmitted by a bluefilter, such as filter 79A, which is used to produce excitation light inthe system light source. This filter transmits light in the wavelengthrange from 370-460 nm or any subset of wavelengths in this range. Of thelight transmitted by this filter, less than 0.001% is in thefluorescence imaging band from 480-750 nm (or whatever desired subsetsof this range is within the specified transmission range of the primaryand reference fluorescence image filters described below).

FIG. 6B illustrates the composition of the light transmitted by a redfilter, such as filter 79B, which is used to produce red reflectancelight in the system light source. This filter transmits light in thewavelength range from 590-750 nm or any subset of wavelengths in thisrange. Light transmitted outside this range should not exceed 1%.

FIG. 6C illustrates the composition of the light transmitted by a greenfilter, such as filter 79C, which is used to produce green reflectancelight in the system light source. This filter transmits light in thewavelength range from 480-570 nm or any subset of wavelengths in thisrange. Light transmitted outside this range should not exceed 1%.

FIG. 6D shows the composition of the light transmitted by a cameraspectral filter, such as filter 118, for defining the primaryfluorescence image in the green spectral band. In this configuration,the filter blocks excitation light and red fluorescence light whiletransmitting green fluorescence light in the wavelength range of 480-570nm or any subset of wavelengths in this range. When used in afluorescence endoscopy video system with the light source filter 79Adescribed above, the filter characteristics are such that any lightoutside of the wavelength range of 480-570 nm, or any desired subset ofwavelengths in this range, contributes no more than 0.1% to the lighttransmitted by the filter.

FIG. 6E shows the composition of the light transmitted by a camerafilter, such as filter 118, for defining the primary fluorescence imagein the red spectral band. In this configuration, the filter blocksexcitation light and green fluorescence light while transmitting redfluorescence light in the wavelength range of 590-750 nm or any subsetof wavelengths in this range. When used in a fluorescence endoscopyvideo system with the light source filter 79A described above, thefilter characteristics are such that any light outside of the wavelengthrange of 590-750 nm, or any desired subset of wavelengths in this range,contributes no more than 0.1% to the light transmitted by the filter.

The operation of the preferred embodiment of the fluorescence endoscopyvideo system will now be described. The cameras 100A as shown in FIGS.4A and 100B as shown in FIG. 4B or 100C as shown in FIG. 4C are capableof operating in color and fluorescence/reflectance imaging modes. Alight source of the type shown in FIG. 2B, that provides a differentoutput every video frame or field is required. In the color imagingmode, the processor/controller 64 provides a control signal to themulti-mode light source 52 that indicates the light source should beoperating in the white light mode and provides a synchronizing signal.The light source 52 sequentially outputs filtered red, green, and bluelight, synchronously with the video field or frame of the image sensors102 and 104. The filtered light from the light source 52 is projectedinto the endoscope light guide 54 and is transmitted to the tip of theendoscope 60 to illuminate the tissue 58.

The processor/controller 64 also protects the sensitive low light imagesensor 104 during color imaging by decreasing the gain of theamplification stage of the sensor. The light reflected by the tissue 58is collected and transmitted by the endoscope image guide 56 to thecamera where it is projected through beamsplitter 106 onto themonochrome image sensor 102, or the light is directly projected throughthe camera beamsplitter 106 onto the monochrome image sensor 102 if thesensor is located within the insertion portion of the endoscope. Theimage projected during each of red, green, blue illuminations istransduced by the monochrome image sensor 102 and the resulting imagesignals are transmitted to the processor/controller 64.

Based on the brightness of the images captured, the processor/controller64 provides a control signal to the multi-mode light source 52 to adjustthe intensity control 80 and thereby adjust the level of light output bythe endoscope light guide 54. The processor/controller 64 may also senda control signal to the camera 100A, 100B or 100C to adjust the gain ofthe monochrome image sensor 102.

The processor/controller 64 interpolates the images acquired duringsequential periods of red, green, and blue illumination to create acomplete color image during all time periods, and encodes that colorimage as video signals. The video signals are connected to color videomonitor 66 for display of the color image. All of the imaging operationsoccur at analog video display rates (30 frames per second for NTSCformat and 25 frames per second for PAL format).

When switching to the fluorescence/reflectance imaging mode, theprocessor/controller 64 provides a control signal to the multi-modelight source 52 to indicate that it should be operating influorescence/reflectance mode. In response to this signal, the lightsource filter wheel 79 stops rotating and the light source 52 selectsand positions the appropriate blue optical filter 79A continuously intothe optical path between the arc lamp 70 and the endoscope light guide54. This change from sequentially changing filters to a static filteroccurs in a period of approximately one second. Filter 79A transmitsonly those wavelengths of light that will induce the tissue 58 underexamination to fluoresce. All other wavelengths of light aresubstantially blocked as described above. The filtered light is thenprojected into the endoscope light guide 54 and transmitted to the tipof the endoscope 60 to illuminate the tissue 58.

As part of setting the system in the fluorescence/reflectance mode, theprocessor/controller 64 also increases the gain of the amplificationstage of the low light image sensor 104. The fluorescence emitted andexcitation light reflected by the tissue 58 are either collected by theendoscope image guide 56 and projected through the camera beamsplitter106 onto the low light image sensor 104 and the image sensor 102, or arecollected and directly projected through the camera beamsplitter 106onto the low light image sensor 104 and the image sensor 102 at theinsertion tip of the endoscope 60. Spectral filter 118 limits the lighttransmitted to the low light image sensor 104 to either green or redautofluorescence light only and substantially blocks the light in theexcitation wavelength band. The autofluorescence image is transduced bythe low light image sensor 104. The reference reflected excitation lightimage is transduced by the monochrome image sensor 102 and the resultingimage signals are transmitted to the processor/controller 64.

Based on the brightness of the transduced images, theprocessor/controller 64 may provide a control signal to the multi-modelight source 52 to adjust the intensity control 80 and thereby adjustthe level of light delivered to the endoscope 60. Theprocessor/controller 64 may also send control signals to the cameras100A, 100B or 100C to adjust the gains of the low light image sensor 104and the monochrome image sensor 102, in order to maintain constant imagebrightness while keeping the relative gain constant.

After being processed, the images from the two sensors are encoded asvideo signals by processor/controller 64. The fluorescence/reflectanceimage is displayed by applying the video signals to different colorinputs on the color video monitor 66.

In order for the combined image to have optimal clinical meaning, for agiven proportion of fluorescence to reference light signals emitted bythe tissue and received by the system, a consistent proportion must alsoexist between the processed image signals that are displayed on thevideo monitor. This implies that the (light) signal response of thefluorescence endoscopy video system is calibrated. One suitablecalibration technique is described in two patent applications previouslyincorporated herein by reference: U.S. patent application Ser. No.09/615,965, and U.S. patent application Ser. No. 09/905,642.

The cameras 100A, 100B, 100C can be operated in a variation of thefluorescence/reflectance mode to simultaneously obtain fluorescenceimages and reflectance images with red, green, and blue illumination.The operation of the system is similar to that described previously forcolor imaging, so only the points of difference from the color imagingmode will be described.

In this variation of the fluorescence/reflectance mode, instead ofchanging from sequential red, green, and blue illumination to staticblue illumination when switching from color imaging tofluorescence/reflectance imaging, the multi-mode light source 52provides the same sequential illumination utilized in the color imagingmode, for all imaging modes. Capture and display of the light reflectedby the tissue is similar to that described previously for the colorimaging mode. However, in addition to the reflectance images captured inthat mode, the gain of the amplification stage of the low light imagesensor 104 is adjusted to a value that makes it possible to captureautofluorescence images during blue illumination. During red and greenillumination, the gain of amplification stage of the low light sensor isdecreased to protect the sensor while the image sensor 102 capturesreflectance images.

In this modified fluorescence/reflectance mode, the camera captures bothreflectance and fluorescence images during the blue illumination period,in addition to reflected light images during the red and greenillumination periods. As for the color imaging mode, the reflectanceimages are interpolated and displayed on the corresponding red, greenand blue channels of a color video monitor to produce a color image.Like the previously described fluorescence/reflectance mode, afluorescence/reflectance image is produced by overlaying thefluorescence image and one or more of the reflectance images displayedin different colors on a color video monitor.

Since individual reflectance and fluorescence images are concurrentlycaptured, both a color image and a fluorescence/reflectance image can bedisplayed simultaneously on the color video monitor. In this case, thereis no need to utilize a separate color imaging mode. Alternatively, asdescribed for the previous version of fluorescence/reflectanceoperation, only the fluorescence/reflectance image may be displayedduring fluorescence/reflectance imaging and a color image displayedsolely in the color imaging mode.

Yet another embodiment of this invention will now be described. Allpoints of similarity with the first embodiment will be assumedunderstood and only points that differ will be described.

In this second embodiment, all aspects of the fluorescence endoscopyvideo system are similar to those of the first embodiment except for thecamera and the light source. A camera 100D for this embodiment of asystem is as shown in FIG. 7A. It differs from the cameras 100A, 100B or100C as described above in that all imaging modes utilize a single, lowlight color image sensor 103 (preferably a color CCD with charge carriermultiplication such as the Texas Instruments TC252) and that nobeamsplitter is required. Alternatively, the color image sensor 103 maybe a three-CCD with charge carrier multiplication color image sensorassembly, a color CCD, a three-CCD color image sensor assembly, a colorCMOS image sensor, or a three-CMOS color image sensor assembly.

Each of the pixel elements on the low light color sensor 103 is coveredby an integrated filter, typically red, green or blue. These filtersdefine the wavelength bands of fluorescence and reflectance light thatreach the individual pixel elements. Such mosaic filters typically haveconsiderable overlap between the red, green, and blue passbands, whichcan lead to considerable crosstalk when imaging dim autofluorescencelight in the presence of intense reflected excitation light. Therefore,a separate filter 118 is provided to reduce the intensity of reflectedexcitation light to the same level as that of the autofluorescence lightand, at the same time, pass autofluorescence light.

In this embodiment, the primary fluorescence and reference images areprojected onto the same image sensor 103, but, because of the individualfilters placed over each pixel, these different images are detected byseparate sensor pixels. As a result, individual primary fluorescence andreference image signals can be produced by processor/controller 64 fromthe single CCD image signal.

In FIG. 7A, light collimating optics 110 is positioned between thetissue 58 and filter 118 and imaging optics 112 is positionedimmediately preceding the color image sensor 103. In an alternativeoptical configuration, camera 100E, as shown in FIG. 7B, eliminates thecollimating optics 110 and imaging optics 112 and replaces them with asingle imaging optics 113 located between the tissue 58 and filter 118.The advantage of this configuration is that all imaging is performed andcontrolled by the same imaging optics 113. The fact that filter 118 islocated in a converging beam path must be considered in specifying thatelement and in the design of the imaging optics.

The operation of a system based on camera 100D of FIG. 7A or 100E ofFIG. 7B will now be described. The cameras 100D and 100E are capable ofoperation in the color, fluorescence/fluorescence, andfluorescence/reflectance imaging modes. For a system based on camera100D or 100E, a light source of the type shown in FIG. 2A, providessteady state output in each imaging mode. As described below, the lighttransmission specifications of the light source filters 76A, 76B, and76C, the filter 118, and the mosaic color filters integrated with theimage sensor 103 are selected such that the intensity of the reflectedlight and fluorescence light at the color image sensor's active elementsresults in transduced image signals with good signal-to-noisecharacteristics and without significant saturation. At the same timethese filters have appropriate light transmission specifications forexcitation and imaging of the primary fluorescence and for colorimaging. The filter transmission characteristics are chosen to providethe desired ratio of relative primary fluorescence to reference lightintensity at the image sensor.

In the color imaging mode, the processor/controller 64 provides acontrol signal to the multimode light source 52 that it should be inwhite light mode. The light source selects and positions the appropriateoptical filter 76A into the optical path between the arc lamp 70 andendoscope light guide 54. Given the presence of filter 118 in cameras100D, 100E which have reduced transmission for excitation light at bluewavelengths, the light source filter 76A should incorporate reducedtransmission at red and green wavelengths to obtain a balanced colorimage at image sensor 103 with the proper proportions of red, green, andblue components.

Image signals from the color low light sensor 103 are processed byprocessor/controller 64. Standard techniques are utilized to produce acolor image from a single color sensor: the image signals from pixelshaving the same filter characteristics are interpolated byprocessor/controller 64 to produce an image signal, related to the passband of each element of the mosaic filter (e.g., red, green, and blue),at every pixel location. The resulting multiple images, which whencombined produce a color image, are encoded by processor/controller 64as video signals. The color image is displayed by connecting the videosignals to the appropriate inputs of color video monitor 66.

Processor/controller 64 also maintains the overall image brightness at aset level by monitoring the brightness of the image signal at each pixeland adjusting the intensity of the light source output and cameraamplifier gains according to a programmed algorithm.

When switching to the fluorescence/fluorescence imaging mode,processor/controller 64 provides a control signal to the multi-modelight source 52 to indicate that it should be influorescence/fluorescence mode. The light source 52 moves light sourcefilter 76B into position in the light beam. Filter 76B transmitsexcitation light and blocks the transmission of light at the green andred fluorescence detection wavelengths, as described below. Thecharacteristics of light source fluorescence excitation filter 76B andexcitation filter 118, along with the mosaic filter elements on thecolor sensor 103, are such that the intensity of blue light at the colorsensor is less than the intensities of red and green autofluorescence atthe sensor, and are such that the ratio of the intensity of redautofluorescence to the intensity of green autofluorescence at the colorsensor 103 has the appropriate value for optimal differentiation betweennormal and abnormal tissue. The fluorescence images are processed, aspreviously described for color imaging, by processor/controller 64 toproduce separate images corresponding to each of the pass bands of themosaic filter (e.g., red, green, and blue). These separate images areencoded as video signals by processor/controller 64. A compositefluorescence/fluorescence image is displayed on the color video monitor66 by applying the video signals from red and green pass bands of themosaic filter to different color inputs of the monitor.

When switching to the fluorescence/reflectance imaging mode,processor/controller 64 provides a control signal to the multi-modelight source 52 to indicate that it should be influorescence/reflectance mode. The light source 52 moves light sourcefilter 76C into position in the light beam. Filter 76C transmits bothexcitation light and reference light and blocks the transmission oflight at fluorescence detection wavelengths, as described below. Thecharacteristics of the light source filter 76C for fluorescenceexcitation and the reflectance illumination and the camera filter 118,along with the mosaic filter on the color sensor 103, as detailed below,are such that the intensity of reflected excitation light at the colorsensor is comparable to the intensity of autofluorescence at the sensor,and should be such that the ratio of the intensity of autofluorescenceto the intensity of reflected reference light at the color sensor 103has the appropriate value. The fluorescence and reflectance images areprocessed, as previously described for color imaging, byprocessor/controller 64 to produce separate images corresponding to eachof the pass bands of the mosaic filter (e.g., red, green, and blue).These separate images are encoded as video signals byprocessor/controller 64. A composite fluorescence/reflectance image isdisplayed on color video monitor 66 by applying the video signals fromthe appropriate mosaic filter pass bands (as discussed below) todifferent color inputs of the monitor.

As indicated above, the filters in the light source and camera should beoptimized for the imaging mode of the camera, the type of tissue to beexamined and/or the type of pre-cancerous tissue to be detected.Although all of the filters described below can be made to order usingstandard, commercially available components, the appropriate wavelengthrange of transmission and degree of blocking outside of the desiredtransmission range for the described fluorescence endoscopy images modesare important to the proper operation of the system. The importance ofother issues in the specification of such filters such as thefluorescence properties of the filter materials and the proper use ofanti-reflection coatings are taken to be understood.

As discussed above, the filters in the light source and camera should beoptimized for the imaging mode of the camera, the type of tissue to beexamined and/or the type of pre-cancerous tissue to be detected, basedon in vivo spectroscopy measurements. The preferred filtercharacteristics for use in the fluorescence endoscopy video systems witha camera of the type shown in FIGS. 7A and 7B, operating in afluorescence/reflectance imaging mode, or a fluorescence/fluorescenceimaging mode, are shown in FIGS. 8A-8F. There are several possibleconfigurations of fluorescence endoscopy video systems, operating in thefluorescence/reflectance imaging mode including green fluorescence withred reflectance, and red fluorescence with green reflectance and red orgreen fluorescence with blue reflectance. The particular configurationutilized depends on the target clinical organ and application. Thefilter characteristics will now be described for each of these fourconfigurations.

FIGS. 8A-8B illustrate a preferred composition of the light transmittedby filters for a color imaging mode. FIG. 8A illustrates the compositionof the light transmitted by the light source filter, such as filter 76A,which is used to produce light for color imaging. The spectral filter118 remains in place during color imaging since there are no movingparts in the present camera embodiment. Accordingly, to achieve correctcolor rendition during color imaging it is necessary for thetransmission of light source filter 76A to be modified, compared to theusual white light transmission for color imaging, such that the lightreceived by the high sensitivity color sensor 103 is white when a whitereflectance standard is viewed with the camera. Therefore, to balancethe effect of spectral filter 118, the transmission of filter 76A in thered and green spectral bands must be less than the transmission in theblue, and the transmission of filter 76A in the blue must extend to along enough wavelength that there is an overlap with the shortwavelength region of appreciable transmission of filter 1118. Filter 76Atransmits light in the blue wavelength range from 370-480 nm or anysubset of wavelengths in this range at the maximum possibletransmission. The transmission of Filter 76A in the green and redwavelength range from 500 nm-750 nm, or any subsets of wavelengths inthis range, is preferably reduced by at least a factor of ten comparedto the transmission in the blue, in order to achieve a balanced colorimage at the high sensitivity color sensor 103, after taking intoaccount the effect of filter 118.

FIG. 8B shows the composition of the light transmitted by the spectralfilter 118, which is used for all imaging modes. In this configuration,the filter blocks the blue excitation light in the range 370-450 nmwhile transmitting red and green light in the wavelength range of470-750 nm or any subsets of wavelengths in this range. When used in afluorescence endoscopy video system in combination with the light sourcefilter 76A described above, the filter characteristics are such that theintensity of light captured by high sensitivity color sensor 103 in thewavelength bands transmitted by the different regions of the sensor'smosaic filter are comparable, when a white reflectance standard isimaged. When used in a fluorescence endoscopy video system forfluorescence/fluorescence imaging in combination with the light sourcefilter 76B described below, the filter characteristics are such that anylight outside of the wavelength range of 470-750 nm (or any desiredsubset of wavelengths in this range) contributes no more than 0.1% tothe light transmitted by the filter.

FIG. 8C illustrates the composition of the light transmitted by afilter, such as filter 76B, which is used to produce excitation light inthe system light source. This filter transmits light in the wavelengthrange from 370-450 nm or any subset of wavelengths in this range. Of thelight transmitted by this filter, preferably less than 0.001% is in thefluorescence imaging band from 470-750 nm (or whatever desired subsetsof this range is within the transmission range of the primary andreference fluorescence wavelength bands defined by the transmission ofthe mosaic filter incorporated in the high sensitivity color sensor103).

FIG. 8D illustrates the composition of the light transmitted by thelight source filter, such as filter 76C, which is used to produce blueexcitation light and red reference light for a green fluorescence andred reflectance imaging mode. This filter transmits light in the bluewavelength range from 370-450 nm, or any subset of wavelengths in thisrange. It also transmits light in the red wavelength range of 590-750nm, or any subset of wavelengths in this range. The light transmitted inthe red wavelength range (or subset of that range) is adjusted, as partof the system design, to be an appropriate fraction of the lighttransmitted in the blue wavelength range. This fraction is selected tomeet the need to match the intensity of the reflected reference lightprojected on the color image sensor to the requirements of the sensor,at the same time as maintaining sufficient fluorescence excitation. Ofthe light transmitted by this filter, less than 0.001% is in the greenwavelength range of 470-570 nm (or whatever desired subset of this rangeis specified as the transmission range of the primary fluorescencewavelength band).

FIG. 8E illustrates the composition of the light transmitted by a lightsource filter which is used to produce excitation light such as filter76C described above for a red fluorescence and green reflectance imagingmode. This filter transmits light in the blue wavelength range from370-450 nm or any subset of wavelengths in this range. It also transmitslight in the green wavelength range of 470-570 nm or any subset ofwavelengths in this range. The light transmitted in the green wavelengthrange (or subset of that range) is adjusted, as part of the systemdesign, to be an appropriate fraction of the light transmitted in theblue wavelength range. This fraction is selected to meet the need tomatch the intensity of the reflected reference light projected on thecolor image sensor to the requirements of the sensor, at the same timeas maintaining sufficient fluorescence excitation. Of the lighttransmitted by this filter, less than 0.001% is in the red fluorescenceimaging wavelength range of 590-750 nm (or whatever desired subset ofthis range is specified as the transmission range of the primaryfluorescence wavelength band).

FIG. 8F illustrates the composition of the light transmitted by a lightsource filter which is used to produce excitation light such as filter76C described above for a red or green fluorescence and blue reflectanceimaging mode. This filter transmits light in the blue wavelength rangefrom 370-470 nm or any subset of wavelengths in this range. The lighttransmitted in the 450-470 nm wavelength range (or subset of that range)is adjusted, as part of the system design, to meet the need to match theintensity of the reflected reference light projected on the color imagesensor to the requirements of the sensor and to provide the appropriateratio of reference reflected light to fluorescence light, at the sametime as maintaining sufficient fluorescence excitation. Of the lighttransmitted by this filter, less than 0.001% is in the fluorescenceimaging wavelength range of 490-750 nm (or whatever desired subset ofthis range is specified as the transmission range of the primaryfluorescence wavelength band).

In an alternative embodiment of the present invention, fluorescence andwhite light images can be obtained by placing a filter on the distal tipof a video endoscope. A limitation of current fluorescencevideoendoscopy systems is that they typically employ the use ofdedicated endoscopes. Fluorescence imaging with such endoscopes isenabled by the use of an excitation barrier filter to block the strongexcitation light used to excite the tissue fluorescence these systemsare intended to image. These barrier filters are built in to theendoscope distal end (typically between the objective lens and the lowlight image sensor) and this built-in filter distinguishes theseendoscopes from those used for conventional videoendoscopy. Thealternative embodiment of the present invention describes the use of anexternally mounted filter in conjunction with a videoendoscope thatcontains a sufficiently sensitive image sensor to image tissuefluorescence. This combination of a conventional video endoscope and anexternally mounted barrier filter can be used in conjunction with anappropriate endoscopic light source to image both in both color andfluorescence modes.

FIG. 9 illustrates one embodiment of a distal filter 202 that can beplaced over the distal end of a conventional white light imagingendoscope to allow the endoscope to perform fluorescence and white lightexaminations of a patient. As will be described below, the filter 202blocks excess excitation light from reaching the image sensor 103 butallows some blue light to pass in order to obtain white light images.

FIGS. 10A and 10B illustrate the operation of the distal filter 202 infurther detail. An endoscope 250 includes illumination ports 252, 254that supply illumination light to an area of interest. A port 256 is thedistal entrance to a working channel through which endoscopic tools canbe passed in order to perform a desired task such as obtaining a biopsysample, marking tissue with dye or perform some other diagnostic ortherapeutic procedures. Light that is reflected from the tissue 350 iscaptured by a lens 258 that images the light onto a low light colorsensor. In this embodiment, the low light sensor comprises a mosaicfilter 260 that is positioned in front of a CCD, CMOS or equivalentimage sensor 262. Image signals produced by the image sensor aretransmitted to an image processor (not shown) that converts the signalsinto video signals that are displayed on a monitor. In addition thevideo signals may be recorded on a video tape, DVD or other storagemedia for later review or comparison.

In order to allow the endoscope to obtain both fluorescence and whitelight images of the tissue 350, the endoscope 250 may be fitted with thedistal end filter 202. In the embodiment shown, the filter 202 isincluded in a filter assembly 300 that does not obscure the illuminationports 252, 254 in order to allow the illumination light to reach thetissue under examination. In addition the filter assembly 300 does notinterfere with other distal tip features including but not limited tofeatures such as water/air nozzles, electrodes, confocal imaging ports,or the working channel 256 so that tools can still be routed through theendoscope. The filter assembly is made of appropriately inert andnon-conductive materials such as plastic or glass or stainless steel, soas also not to interfere with the particular material or electricalcharacteristics of the endoscope tip that may be required for use inconjunction with endotherapy techniques, such as argon ion plasmacoagulation (APC), electrocautery, cryotherapy, or photodynamic therapy(PDT)

The filter 202 is positioned in front of the imaging lens 258 andprevents excitation light from reaching the image sensor. In oneembodiment, the filter removes the excitation light having wavelengthsin the range of 370-450 nm. or some subset of this range but passes someblue light, for example 451-480 nm. for use in white light imaging aswill be described below. Because most endoscope have objective lenseswith a wide field of view, the filter should block excitation light overa corresponding wide field of view and over the range of angles ofincidence of light collected by the endoscope objective. The filtershould also be thin enough not to introduce optical aberrations orinterfere with the mechanical properties and maneuverability of theendoscope tip. Dye-based absorption filters that block the desired rangeof excitation light and operate over a wide field of view may thereforebe preferred for this application. Specific examples of such filtersinclude Kodak Wratten gel filters. For durability, the filter ispreferably constructed in a manner similar to optically protectiveeyewear such as laser goggles. As shown in FIG. 10A, the distal filter202 allows fluorescence images to be obtained in at least two modes. Ina first mode, excitation light, typically in the blue wavelengthspectral band, is provided from a light source (not shown) and throughthe illumination ports 252, 254 whereupon it strikes the tissue sample350. A portion of the excitation light is absorbed by the tissue andcauses a tissue sample to produce fluorescence light while a portion ofthe excitation light is reflected off the surface of the tissue 350.Excitation light reflecting from the surface of the tissue 350 isblocked by the filter 202 in the filter assembly 300, while thefluorescence light and light in other spectral bands passes through thefilter 202.

In one fluorescence imaging mode, only fluorescence light is used toproduce video images of the tissue. In another mode, the tissue isilluminated with the excitation light and some amount of reflectancelight. As shown in FIG. 10A, the reflectance light is in the redspectral band. The reflectance light passes through the filter 202 andthe filter 260 in front of the image sensor 262. Images of the tissueare obtained by combining for example, the green pixels from the imagesensor 262 to obtain a fluorescence image in the green spectral band andthe red pixels from the image sensor 262 to obtain a reflectance imagein the red spectral band. Alternatively, the reflectance light may beprovided in the green spectral band and a red fluorescence imageobtained. In yet another embodiment, blue reflectance light havingwavelengths that are not filtered by the filter 202 can be used toproduce the reflectance image. The fluorescence and reflectance imagesmay be combined for display for a viewer. In order to prevent thereflectance image from overpowering the fluorescence image, the amountof reflectance light supplied from the light source is selected to becomparable to the amount of fluorescence light received by the imagesensor 262.

As shown in FIG. 10B, the endoscope 250 can also be used to obtain whitelight images of the tissue 350 by illuminating the tissue with lightincluding red, green and blue spectral components. The illuminationlight is reflected by the tissue sample 350, passes through the filter202 and is focused onto the filter 260 by the endoscope's imaging lens258. Because the filter 202 removes most of the blue reflectance light,the light received by the image sensor 262 includes excess red and greenlight. To compensate for the reduced blue light at the image sensor, theillumination light should either contain excess blue light in the bandthat passes through the filter 202 or contain reduced green and redlight so that the resulting image produced by the image sensor may bewhite balanced. Additional fine tuning of the white balance of theimages produced by the image sensor 262 can be accomplished by imageprocessing software in the image processing board (not shown).

One embodiment of a filter assembly 200 that is placed on the distal tipof an endoscope is shown in FIG. 11. A filter 202 may be mounted in aframe that is snap-fitted over the distal end face of an endoscope. Thefilter and/or frame are secured to the endoscope with a mechanical,adhesive, magnetic force or other means. In one embodiment, the frameincludes a set of proximally extending tabs 204, 206 that are secured tothe outer circumference of the endoscope tip.

The filter 202 is positioned in front of the imaging lens of theendoscope. As indicated above, the filter 202 operates to remove asubstantial portion of reflected blue excitation light used during thefluorescence imaging mode. In the embodiment shown in FIG. 11, the framealso includes a hole 210 that is positioned over the working channel ofthe endoscope in order to allow tools, such as biopsy forceps, snares,cytology brushes, etc., to be passed through the working channel.Positioned on either side of the frame are holes 212, 214 that allowlight from the illumination guides to pass through the frame.

Another embodiment of the present invention also utilizes a filter onthe distal tip of a video endoscope. Some videoendoscope systems do notemploy the use of color image sensors and instead use sequentialillumination in various spectral (e.g blue, green, red) bands and thencombine monochrome images to produce full color images. An externalbarrier filter 202 can be used in an endoscope of the type shown in FIG.9 with the high sensitivity color sensor 103 replaced by a monochromeimage sensor and sequential illumination of the type discussed inconnection with FIG. 2B.

Briefly, in accordance with one embodiment of the invention,autofluorescence images are obtained by illuminating with an excitationlight. Color images are obtained by sequential illumination with red,green and blue light; wherein the blue light either includes wavelengthsor is limited to wavelengths not blocked by the filter 202.

The fluorescence endoscopy video systems described in the aboveembodiments have been optimized for imaging endogenous tissuefluorescence. They are not limited to this application, however, and mayalso be used for photo-dynamic diagnosis (PDD) applications. Asmentioned above, PDD applications utilize photo-active drugs thatpreferentially accumulate in tissues suspicious for early cancer. Sinceeffective versions of such drugs are currently in development stages,this invention does not specify the filter characteristics that areoptimized for such drugs. With the appropriate light source camerafilter combinations, however, a fluorescence endoscopy video systemoperating in either fluorescence/fluorescence orfluorescence/reflectance imaging mode as described herein may be used toimage the fluorescence from such drugs.

As will be appreciated, each of the embodiments of a camera for afluorescence endoscopy video system described above, due to theirsimplicity, naturally lend themselves to miniaturization andimplementation in a fluorescence video endoscope, with the camera beingincorporated into the insertion portion of the endoscope. The camerascan be utilized for both color imaging and fluorescence imaging, and intheir most compact form contain no moving parts.

1. A filter assembly for use with an imaging endoscope, comprising: afilter adapted to be positioned in front of an imaging lens of theendoscope, the filter blocking excitation light from reaching an imagesensor of the endoscope so that the image sensor can producefluorescence images, the filter also passing an amount of blue light sothat the image sensor can produce white light images of the tissue. 2.The filter assembly of claim 1, wherein the filter blocks excitationlight having wavelengths in a range of 370 to 450 nm.
 3. The filterassembly of claim 2, wherein the filter passes light having wavelengthsin a range of 450-480 nm.
 4. The filter assembly of claim 1, furthercomprising means for securing the filter to a distal end of theendoscope.
 5. The filter assembly of claim 4, wherein the means forsecuring the filter includes one or more tabs that are secured to theendoscope.
 6. The filter assembly of claim 4, wherein the means forsecuring the filter includes an adhesive.
 7. The cap of claim 4, whereinthe means for securing the filter includes a magnet.
 8. The cap of claim1, wherein the means for securing the filter includes a sleeve that fitsover a distal end of the endoscope.
 9. A system for producingfluorescence and white light images with an imaging endoscope thatincludes an image sensor, comprising: a removable filter that is placedin front of an imaging lens of the endoscope that filters excitationlight from reaching the image sensor and passes an amount of blue light;an image processor that produces images from image signals received fromthe image sensor; a light source for producing excitation light for usein a fluorescence imaging mode and illumination light for use in a whitelight imaging mode, wherein the illumination light produced by the lightsource compensates for the effects of the filter so that the imagesproduced from the image sensor can be white balanced.
 10. The system ofclaim 9, wherein the illumination light produced by the light source hasan increased amount of blue light in a wavelength band that passesthrough the filter compared with an amount of red and green light in theillumination light.
 11. The system of claim 9, wherein the illuminationlight produced by the light source has a decreased amount of green andred light so that that the red, green and blue light received at theimage sensor have substantially equal magnitudes.
 12. The system ofclaim 9, wherein the light source produces an amount of reflectancelight when operating in a fluorescence imaging mode.
 13. The system ofclaim 12, wherein the reflectance light is red light.
 14. The system ofclaim 12, wherein the reflectance light is green light.
 15. The systemof claim 12, wherein the reflectance light is blue light havingwavelengths that pass through the filter.
 16. The system of claim 9,wherein the image sensor is a color image sensor and the illuminationlight includes red, green and blue spectral components.
 17. The systemof claim 9, wherein the image sensor is a monochrome sensor and theillumination light includes sequentially produced red, green and bluespectral components.